The pulsation modes of the pre-white dwarf PG 1159-035

Context. PG 1159-035 , a pre-white dwarf with $T_{{\rm eff}}$$\simeq$ 140 000 K, is the prototype of both two classes: the PG 1159 spectroscopic class and the DOV pulsating class. Previous studies of PG 1159-035 photometric data obtained with the Whole Earth Telescope (WET) showed a rich frequency spectrum allowing the identification of 122 pulsation modes. Analyzing the periods of pulsation, it is possible to measure the stellar mass, the rotational period and the inclination of the rotation axis, to estimate an upper limit for the magnetic field, and even to obtain information about the inner stratification of the star. Aims.We have three principal aims: to increase the number of detected and identified pulsation modes in PG 1159-035 , study trapping of the star's pulsation modes, and to improve or constrain the determination of stellar parameters. Methods.We used all available WET photometric data from 1983, 1985, 1989, 1993 and 2002 to identify the pulsation periods. Results.We identified 76 additional pulsation modes, increasing to 198 the number of known pulsation modes in PG 1159-035 , the largest number of modes detected in any star besides the Sun. From the period spacing we estimated a mass $M/M_\odot$ = 0.59 $\pm$ 0.02 for PG 1159-035 , with the uncertainty dominated by the models, not the observation. Deviations in the regular period spacing suggest that some of the pulsation modes are trapped, even though the star is a pre-white dwarf and the gravitational settling is ongoing. The position of the transition zone that causes the mode trapping was calculated at $r_{\rm c}/R_\star$ = 0.83 $\pm$ 0.05. From the multiplet splitting, we calculated the rotational period $P_{\rm rot}$ = 1.3920 $\pm$ 0.0008 days and an upper limit for the magnetic field, B < 2000 G. The total power of the pulsation modes at the stellar surface changed less than 30% for $\ell$ = 1 modes and less than 50% for $\ell$ = 2 modes. We find no evidence of linear combinations between the 198 pulsation mode frequencies. PG 1159-035 models have not significative convection zones, supporting the hypothesis that nonlinearity arises in the convection zones in cooler pulsating white dwarf stars

Whole Earth Telescope observations of BPM 37093: a seismological test of crystallization theory in white dwarfs

BPM 37093 is the only hydrogen-atmosphere white dwarf currently known which has sufficient mass (~1.1 MO) to theoretically crystallize while still inside the ZZ Ceti instability strip (Teff ~ 12 000 K). As a consequence, this star represents our first opportunity to test crystallization theory directly. If the core is substantially crystallized, then the inner boundary for each pulsation mode will be located at the top of the solid core rather than at the center of the star, affecting mainly the average period spacing. This is distinct from the “mode trapping” caused by the stratified surface layers, which modifies the pulsation periods more selectively. In this paper we report on Whole Earth Telescope observations of BPM 37093 obtained in 1998 and 1999. Based on a simple analysis of the average period spacing we conclude that a large fraction of the total stellar mass is likely to be crystallized